8. String to Integer (atoi)

Implement atoi to convert a string to an integer.

Hint: Carefully consider all possible input cases. If you want a challenge, please do not see below and ask yourself what are the possible input cases.

Notes: It is intended for this problem to be specified vaguely (ie, no given input specs). You are responsible to gather all the input requirements up front.

Update (2015-02-10):
The signature of the C++ function had been updated. If you still see your function signature accepts a const char * argument, please click the reload button to reset your code definition.

spoilers alert… click to show requirements for atoi.

public class Solution {  
    public int myAtoi(String str) {  
          
        // 1. null or empty string  
        if (str == null || str.length() == 0) return 0;  
          
        // 2. whitespaces  
        str = str.trim();  
          
        // 3. +/- sign  
        boolean positive = true;  
        int i = 0;  
        if (str.charAt(0) == '+') {  
            i++;  
        } else if (str.charAt(0) == '-') {  
            positive = false;  
            i++;  
        }  
          
        // 4. calculate real value  
        double tmp = 0;  
        for ( ; i < str.length(); i++) {  
            int digit = str.charAt(i) - '0';  
              
            if (digit < 0 || digit > 9) break;  
              
            // 5. handle min & max  
            if (positive) {  
                tmp = 10*tmp + digit;  
                if (tmp > Integer.MAX_VALUE) return Integer.MAX_VALUE;  
            } else {  
                tmp = 10*tmp - digit;  
                if (tmp < Integer.MIN_VALUE) return Integer.MIN_VALUE;  
            }  
        }  
          
        int ret = (int)tmp;  
        return ret;  
    }  
}  

Java2

class Solution {
    public int myAtoi(String str) {
        str = str.trim();
        int retVal = 0;
        int startIndex = 0;
        int sign = 1;
        if(str.equals("")) {
            return 0;
        }
        startIndex = str.charAt(0) == '-' || str.charAt(0) == '+' ? 1 : 0;
        sign = str.charAt(0) == '-' ? -1 : 1; 
        for(int i = startIndex ; i < str.length() ; i++) {
            char c = str.charAt(i);
            if('0' <= c && c <= '9') {
                if(retVal > (Integer.MAX_VALUE - (c - '0')) / 10)
                    return sign > 0 ? Integer.MAX_VALUE : Integer.MIN_VALUE;
                retVal = retVal * 10 + (c - '0');
            } else {
                break;
            }
        }
        return sign * retVal;
    }
}

JS

/**
 * @param {string} str
 * @return {number}
 */
var myAtoi = function(str) {
    str = str.trimLeft(); // remove whitespace from beginning of string

    if (!/[\d+-]/.test(str[0])) return 0; // if first character is not +,- or a digit

    const number = Number(str.match(/[+-]?\d*/)); // regex means: optional(+,-) follows by zero (case " +  514 " => expect 0) or more digits
    if (Number.isNaN(number)) return 0; // invalid integral number. eg: " +  514" => expect 0 (Number('+') => NaN)

    const MAX_INT = 2 ** 31 - 1;
    const MIN_INT = - (2 ** 31);
    if (number < MIN_INT) return MIN_INT;
    if (number > MAX_INT) return MAX_INT;
    return number;
};